NEOGENE TECTONIC EVOLUTION OF THE MAGDALENA SHELF

The Magdalena fan is an apparently beheaded submarine depocenter that has figured prominently in Middle to late Miocene reconstructions of Pacific-North American plate interactions. The deposit accumulated rapidly at the base of the continental slope atop newly formed oceanic crust of the Farallon plate from 14.5-13 Ma. Subduction of this crust ceased as the Pacific-Farallon spreading center encountered the trench. The widely accepted two-phase kinematic model for the formation of the Gulf of California holds that ~300 km of dextral shear between the Pacific and North America plates occurred along faults west of Baja California prior to the onset of dextral-transtensional shearing in the Gulf at ~6 Ma. However, detrital zircon U-Pb ages strongly indicate a western Baja source region for the fan and imply that cumulative dextral slip along faults west of Baja was < 150 km. Previous models held that the fan was fed by a major extraregional river system that traversed the mouth of the protogulf. We alternatively regard fan formation as reflecting enhanced activity of the same depositional processes that infilled the formerly active trench along the entire strike length of the Magdalena microplate. Specifically, we envision that the ridge-trench collision and underplating of buoyant nascent oceanic crust triggered erosional denudation of the Magdalena shelf along western Baja and caused sediment to spill over the continental shelf. Previously recognized geologic relationships and new low-temperature thermochronology (K-feldspar MDD, and apatite and zircon fission track and (U-Th)-He analysis) indicate that the Early Middle Miocene topography in the region was controlled by a NNE-trending buttress unconformity established east of the Magdalena shelf that followed the western margin of the present Los Cabos block and extended northwards into the present Gulf. Detrital zircon age distributions within the fan are easily accounted for by reworking of Magdalena shelf Tertiary strata (Eocene Tepetate Fm., Miocene Comundu Group) with (or without) additional input of basement-derived detritus from exposed basement further east. The fan's source shut off when normal faults west of Baja California began to accommodate transtensional shearing and form rift basins that captured detritus that previously reached the trench.